Comments are those little notes underneath items. You can see them on user-created items except for games. They let the people buying your items give you feedback, such as how awesome your shirt is.
Feedback and chat
Comments are used for a variety of things, from just chatting to giving information about a place to random things to say. Take a look at that one picture in the corner, you can see one person saying how the cars are awesome, and I replied with some information about Bug log in the place. This is what comments are really meant for.
Posting a new comment
If you want to make your voice heard just write in your message into the Comment on this Item box and hit Submit. You can only post every so often to make sure people aren't spamming items. Also there is a character limit. Comments are meant to be short!
Sometimes when looking through Comments you'll see things like "Post this on 5 hats to get a million tix!" or "If you are reading this you will die in the next hour post this on 2 more badges to save your life!". These are what's called Chain Spam. This is a type of spam where you post the message on more places, which makes more people see it, and post it more, and more, and more until it's everywhere and everyone is posting it. The messages are fake, you can't get tickets like that, nor will anything deadly happen if you post the message around in a few places. All it does is make people mad at you for spamming.
Bugs and glitches
It's possible to make a page improperly load and you will see the players username as username and the comment text as text.